Marine Chemistry 128-129 (2012) 64–71
Contents lists available at SciVerse ScienceDirect
Marine Chemistry
journal homepage: www.elsevier.com/locate/marchem
Absorption and fluorescence of dissolved organic matter in submarine hydrothermal
vents off NE Taiwan
Liyang Yang a, b, Huasheng Hong b, Weidong Guo b, Chen-Tung Arthur Chen a,⁎, Pei-I Pan a, Chun-Chin Feng a
a
b
Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung 804, Taiwan, ROC
State Key Laboratory of Marine Environmental Science, College of Oceanography and Environmental Science, Xiamen University, Xiamen, Fujian 361005, PR China
a r t i c l e
i n f o
Article history:
Received 22 November 2010
Received in revised form 22 August 2011
Accepted 7 October 2011
Available online 19 October 2011
Keywords:
Hydrothermal vent
DOM
CDOM
EEM
Humification index
Autochthonous index
a b s t r a c t
The role of hydrothermal vents as either a source or a sink for chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the oceans is unknown, since DOM absorption and fluorescence have not
been reported for submarine hydrothermal vents. Water samples were collected from two shallow submarine hydrothermal vents off NE Taiwan, the white vent and the yellow vent, during two cruises in August,
2010. Absorption and excitation-emission-matrix fluorescence spectroscopy were used to characterize the
optical properties of DOM from such extremely special environments. The absorption coefficients at wavelength 300 nm (a300) were much higher at the white vent mouth and 1 m below it (2.52 ± 0.88 m -1) than
in the background (0.34 ± 0.12 m -1). This indicated that the white vent was a source of CDOM for seawater.
Three fluorescent components were identified using parallel factor analysis: humic-like C1, tyrosine-like C3,
and C2 as a combination of tryptophan-like and marine humic-like components. Both C1 and C3 (but not C2)
had their highest fluorescence intensity at the white vent mouth and 1 m below it, suggesting the role of the
vent as a source for both humic-like and tyrosine-like DOM. Samples from the yellow vent mouth also had a
higher a300 than the ambient seawater in our first cruise, but had fluorescence intensities of C(1–3) similar to
the ambient seawater. Overall, the low humification index (HIX: 1.40 ± 0.30) and the high autochthonous
index (BIX: 1.27 ± 0.63) indicated that the DOM likely had low humic contents and was mainly autochthonous of biological or bacterial origin in the study area. A biplot of HIX and BIX showed that DOM from the hydrothermal vents had a characteristic similar to terrestrial cave and spring waters, but was distinct from
isolated humics.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Hydrothermal systems serve as a major source or sink for a number
of elements in the oceans, such as Mg, S, Li, Rb, Fe and Mn (Elderfield
and Schultz, 1996; Chen et al., 2005a). There is massive metal sulfide
precipitation from the hydrothermal fluids at and just below the seafloor in some deep-sea vent systems (Herzig and Hannington, 2006).
Biota (e.g., over 500 species of macrofauna) are also reported for a
number of hydrothermal vents worldwide (Tarasov et al., 2005). In
contrast, much less study is focused on dissolved organic matter
(DOM) in the hydrothermal vents (Svensson et al., 2004; Lang et al.,
2006; McCarthy et al., 2011; Chen, 2011), although it plays an important role in many biogeochemical processes in aquatic environments.
For example, it can affect the speciation, solubility, bioavailability and
toxicity of metals (e.g., Sander and Koschinsky, 2011) and is a nutrient
and energy source for heterotrophic bacteria (e.g., Fellman et al., 2010;
Lønborg et al., 2010).
⁎ Corresponding author. Tel.: + 886 7 5255136; fax: + 886 7 525 5346.
E-mail address: [email protected] (C.-T.A. Chen).
0304-4203/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.marchem.2011.10.003
In deep-sea hydrothermal systems, dissolved organic carbon (DOC)
is added when seawater passes through low-temperature vents, but is
removed when seawater passes through high-temperature vents and
off-axis vents (Lang et al., 2006). The estimated flux for each type of
vent (0.7–230 × 1010 g C yr− 1) is minor compared to other oceanic
sources and sinks (e.g., 2.5 × 1014 g C yr− 1 from river input, Lang et al.,
2006). Recently, McCarthy et al. (2011) found that DOC in ridge-flank
fluids have very old radiocarbon ages of 11,800–14,400 years before
13
the present and have δ C values ranging from −26‰ to −35‰, suggesting that DOC in such extreme environments is synthesized by
chemosynthetic microbes using the old dissolved inorganic carbon in
the fluids. The shallow hydrothermal system of Vulcano Island (Italy)
is well studied for DOC, volatile fatty acids, amino acids and neutral
aldoses (Svensson et al., 2004; Skoog et al., 2007). The high concentrations and the composition characteristics of dissolved amino acids,
indicate that the DOM is likely to be labile and fresh in these shallow
hydrothermal sites (Svensson et al., 2004). However, to the best of
our knowledge, DOM absorption and fluorescence have not yet been
reported for submarine hydrothermal systems.
Absorption and fluorescence measurements provide information on
the concentration and composition of chromophoric and fluorescent
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
DOM (i.e., CDOM and FDOM). Absorption coefficient and fluorescence
intensity serve as concentration indicators of CDOM and FDOM, while
the absorption spectral slope and fluorescence indices (such as the
humification index, HIX and the autochthonous index, BIX) are useful
for assessing DOM composition (e.g., Hong et al., 2005; Guo et al.,
2007; Helms et al., 2008; Huguet et al., 2009; Birdwell and Engel,
2010; Ortega-Retuerta et al., 2010). Furthermore, excitation-emissionmatrix (EEM) fluorescence spectroscopy and parallel factor analysis
(PARAFAC) are powerful in identifying the different fluorescent components of DOM and assessing their sources and dynamics in freshwater,
seawater, sea ice, aerosols and soil (e.g., Stedmon and Markager,
2005; Murphy et al., 2008; Jaffé et al., 2008; Mladenov et al., 2009;
Yamashita et al., 2010; Guo et al., 2011; Jørgensen et al., 2011).
Kueishantao islet is a volcanic island off NE Taiwan, with >30
shallow submarine hydrothermal vents adjacent to the islet (Chen
et al., 2005a, b). There are two typical kinds of hydrothermal vents
in this area, i.e., yellow vents and white vents (Chen et al., 2005a,
b). The inorganic geochemistry of hydrothermal fluids and the deposits, gas compositions and biota are well studied (e.g., Chen et al.,
2005a), while little is known about the DOM in this area. Therefore,
we aimed to: (1) study the concentration and composition of CDOM
and FDOM in the submarine hydrothermal vents using absorption
spectroscopy, EEM fluorescence spectroscopy-PARAFAC and fluorescence indices; and (2) examine whether the hydrothermal vents act
as a source of CDOM or FDOM for the ambient seawater.
2. Materials and methods
2.1. Study area
65
Fig. 1. Study area and sampling stations.
were filtered through pre-combusted 0.7 μm GF/F filters and stored
in a freezer for DOC measurements. Samples were filtered through
acid-rinsed 0.2 μm Millipore polycarbonate filters and stored in the
cold (4 °C) and dark for CDOM and FDOM measurements. The filtration was carried out within several hours after sample collection in
the first cruise and within 1 day in the second cruise, while the optical
measurements were carried out within 4–9 days. However, visible
white particles appeared in the filtrates which were collected from
the hydrothermal field and stored in the cold for optical measurements, and hence all the samples were re-filtered immediately before
the optical measurements were made. These white precipitates might
have removed CDOM/FDOM and led to underestimation of the role of
hydrothermal vents as a CDOM/FDOM source for the seawater. However,
this effect was likely limited, since the amount of the precipitate in the
filtrates was low.
Temperature was measured either in situ with a thermocouple at
the vent mouths, 1 m below them, and 3 m above them (Table 1,
Chen et al., 2005a), or for collected samples using a thermometer.
Salinity was determined using a Guildline 8400B Autosal Laboratory
Salinometer, while pH was measured using a Radiometer PHM-85
pH meter at 25 °C (Chen et al., 2005a).
Kueishantao islet (121°57′E, 24°50′N) is one of the many small
volcanic islands off NE Taiwan at the tectonic junction of the fault system extension of Taiwan and the southern rifting end of the Okinawa
Trough. There are >30 hydrothermal vents over an area of ~0.5 km 2
east of the islet at a water depth b30 m, emitting hydrothermal fluids
and volcanic gases (Chen et al., 2005a, b). Yellow vents discharging
elemental sulfur particles have shown temperatures of 78–116 °C
(mean: 106 ± 9 °C) and pH values of 1.52–6.32 (mean: 2.49 ± 0.72),
while vents discharging whitish fluids have lower temperatures of
30–65 °C (mean: 51 ± 8 °C) but higher pH values of 1.84–6.96
(mean: 3.20 ± 1.17). Their dry gas compositions are dominated by
CO2 (>92%). Evidence from sulfur and helium isotopes, Mg and SiO2
indicate that the hydrothermal fluids originate mainly from the
upper mantle. The close correlation between vent temperature and
diurnal tides suggest a rapid circulation of hydrothermal fluids
above the magma. There are massive sulfur deposits around the
vents, especially the yellow vents. A few benthic, algal and fish species are also found near the vents (Chen et al., 2005a, b).
DOC concentration was measured using the method of high temperature catalytic oxidation after removing inorganic carbon by acidification and oxygen purging, using a high TOCIIanalyzer (Elementar,
Germany). Each sample was analyzed twice with an analytical precision within 5% (mostly b4%). A five-point calibration was carried out
using solutions of potassium hydrogen phthalate as standards. The accuracy of the measurements was verified with Low Carbon Water and
Deep Sea Water (from Dr. D. A. Hansell, University of Miami). The
measured DOC concentration of Deep Sea Water (46.6 ± 1.6 μmol L − 1,
n = 11) was close to the recommended values (44–46 μmol L − 1).
2.2. Sample collection
2.4. Absorption measurements
Samples were collected during two cruises on August 3rd–5th and
24th–27th, 2010 (Fig. 1). A yellow vent (station Y0, water depth:
8.5 m) and a white vent (station W0, water depth: 16.2 m) were sampled at four depths (1 m below the vent mouth, at the vent mouth,
3 m above the vent mouth and at the sea surface). On August 3rd–
5th, surface waters were also collected from two nearby sites (W1
and Y1) and two reference stations (W2 and Y2, clear water without
visible particles). On August 26th, samples were collected from surface waters at stations A–C and from 2–4 depths at stations D–J. On
August 27th, surface waters were collected at stations 1–15.
Glass syringes connected with 1.5 m-long Teflon pipe through
stainless steel triple valves were used for sampling in and above the
vents. Other samples were collected using Niskin bottles. All the samples were transferred into acid-rinsed and pre-combusted brown
glass bottles and stored in the cold and dark. Back on land, samples
CDOM absorption spectra were scanned using a Cary 50 UV–vis
spectrometer from 200 nm to 800 nm (every 1 nm) at a scan rate of
60 nm min − 1. Milli-Q water was used as the blank. Absorbance (A)
at each wavelength (λ) was baseline corrected by subtracting the
mean absorbance from 700–800 nm, and then converted to absorption coefficient (a) as aλ = 2.303 Aλ/l, where l is the path length
(i.e., 0.05 m) (Guo et al., 2007). In our study absorption coefficient
at a wavelength of 300 nm (a300) was used to represent the CDOM
concentration. The spectral slope ratio SR (the ratio of the slope over
275–295 nm to that over 350–400 nm) was calculated, which is
negatively correlated with the molecular weight (Helms et al., 2008).
Typically, aλ decreased exponentially with λ while ln(aλ) correlated linearly with λ. However, two samples from 1 m below the yellow
vent mouth did not fit that pattern, and their aλ values increased
much more rapidly with decreasing λ when λ b 400 nm due to an
2.3. DOC measurements
66
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
Table 1
Results of two cruises around the shallow submarine hydrothermal vents off NE Taiwan (T: temperature; S: salinity; a300: CDOM absorption coefficient at the wavelength of
300 nm; SR: CDOM absorption spectral slope ratio; C(1–3): maximum fluorescence intensity of the three fluorescent components; HIX: humification index; BIX: autochthonous
index).
S
pH
DOC (μM)
a300 (m− 1)
SR
C1 (RU)
C2 (RU)
C3 (RU)
HIX
BIX
First cruise (August 3rd–5th, 2010)
Y0
0
31
5.5
29c
a
8.5
51c
9.5b
105c
W0
0
30
13
35c
16.2a
35c
17.2b
41c
Y1
0
31
Y2
0
33
W1
0
29
W2
0
30
33.6
33.4
33.6
33.7
33.5
33.4
33.2
32.8
33.7
33.5
33.6
33.7
5.93
5.89
5.25
2.82
5.91
5.16
5.19
4.83
8.07
8.08
6.09
7.95
64
71
70
70
77
69
66
62
64
73
82
66
0.49
1.00
1.04
39.1e
0.79
0.79
2.89
1.75
0.38
0.49
0.61
0.38
1.61
1.79
2.14
\e
1.12
0.78
2.08
1.81
1.72
1.29
1.17
2.64
0.040
0.030
0.032
\e
0.037
0.040
0.054
0.044
0.027
0.030
0.042
0.028
0.075
0.054
0.043
\e
0.029
0.027
0.037
0.037
0.048
0.101
0.036
0.065
0.038
0.033
0.036
\e
0.048
0.047
0.077
0.062
0.020
0.049
0.050
0.023
1.53
1.34
1.21
\e
1.16
1.36
0.97
1.12
2.06
1.21
1.31
1.71
2.01
1.77
1.24
\e
0.75
1.09
1.37
0.93
1.67
1.94
1.18
1.57
Second cruise (August 24th–27th, 2010)
Y0
0
\d
5
39c
9.5b
97c
W0
0
\d
14
33c
17a
44c
18b
58c
A
0
29
B
0
29
C
0
29
D
0
29
3
29
7
29
10
30
E
1
29
5
29
12
29
F
1
30
6
29
G
1
29
5
29
10
29
H
1
30
5
30
10
29
18
29
I
1
30
5
29
15
28
30
28
J
1
29
5
29
15
29
30
28
1
0
31
2
0
29
3
0
30
4
0
29
5
0
29
6
0
29
7
0
29
8
0
29
9
0
29
10
0
30
11
0
29
12
0
29
13
0
29
14
0
29
15
0
31
33.9
33.3
33.1
33.7
33.7
33.1
33.0
33.8
33.8
33.8
33.8
33.8
33.8
33.7
33.8
33.7
33.8
33.7
33.7
33.7
33.7
33.7
33.7
33.8
33.7
33.7
33.7
33.8
33.8
33.8
33.8
33.8
33.7
33.8
33.8
33.8
33.8
33.7
33.8
33.8
33.7
33.7
33.8
33.8
33.8
33.8
33.8
34.1
33.9
6.90
2.62
2.22
6.78
7.40
5.84
5.74
7.96
7.95
7.98
7.99
7.94
7.94
7.88
7.38
7.37
7.74
6.43
6.73
6.41
6.86
6.86
6.65
7.04
7.38
7.25
7.35
7.92
7.92
7.91
7.95
6.35
7.98
7.95
8.00
7.20
7.04
8.01
7.26
8.02
8.03
7.04
7.27
7.98
7.12
7.99
8.02
8.02
8.03
95
77
\d
\d
87
\d
\d
\d
86
81
76
77
79
79
83
93
79
78
79
75
72
80
74
79
72
76
78
73
71
70
73
69
72
69
76
72
74
70
70
72
74
99
72
60
79
75
73
80
\d
0.39
0.43
23.5e
0.73
0.50
1.85
3.58
0.69
0.40
0.31
0.08
0.20
0.15
0.11
0.23
0.19
0.26
0.28
0.29
0.25
0.34
0.42
0.33
0.34
0.36
0.29
0.44
0.42
0.40
0.52
0.30
0.34
0.35
0.27
0.32
0.47
0.45
0.30
0.28
0.28
0.36
0.43
0.45
0.34
0.43
0.41
0.48
0.38
0.50
1.30
\f
\e
\f
0.79
1.56
1.60
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
\f
0.81
\f
1.10
0.53
\f
\f
\f
\f
\f
0.73
0.69
1.08
\f
1.30
\f
1.49
\f
\f
\f
\f
0.67
\f
\f
\f
\g
0.039h
\e
0.041
0.048
0.086
\g
0.029
0.022
0.026
0.028
0.033
0.026
0.029
0.057
0.028
0.028
0.031
0.030
0.035
0.034
0.043
0.033
0.034
0.037
0.036
0.041
0.042
0.039
0.040
0.033
0.025
0.030
0.034
0.036
0.035
0.039
\g
0.031
0.030
0.030
0.038
0.035
0.029
0.048
0.029
0.028
0.042
0.048
\g
0.078h
\e
0.037
0.029
0.086
\g
0.023
0.015
0.022
0.033
0.022
0.013
0.014
0.035
0.016
0.015
0.017
0.049
0.033
0.019
0.059
0.047
0.021
0.026
0.020
0.015
0.020
0.017
0.035
0.026
0.016
0.016
0.016
0.024
0.019
0.025
\g
0.017
0.016
0.015
0.037
0.018
0.013
0.043
0.017
0.015
0.044
0.042
\g
0.033h
\e
0.066
0.052
0.130
\g
0.050
0.026
0.028
0.035
0.041
0.037
0.037
0.037
0.033
0.039
0.046
0.062
0.055
0.046
0.051
0.035
0.035
0.039
0.042
0.046
0.041
0.041
0.044
0.037
0.035
0.038
0.033
0.058
0.046
0.058
\g
0.043
0.035
0.044
0.045
0.046
0.041
0.051
0.048
0.043
0.055
0.063
0.83
0.91h
\e
1.13
1.64
0.96
0.66
1.34
2.01
1.67
1.39
1.54
1.39
1.15
1.88
1.76
1.32
1.12
0.53
1.14
1.27
1.40
1.46
1.54
1.44
1.59
1.81
1.82
1.87
1.58
1.32
1.34
1.63
1.83
1.32
1.43
1.45
1.91
1.36
1.47
1.41
1.52
1.41
1.42
1.61
1.20
1.42
1.38
1.50
3.96
1.88h
\e
1.70
0.77
1.52
2.80
0.71
1.01
0.52
1.36
1.03
0.90
1.21
0.61
1.28
1.17
1.06
1.29
1.72
1.12
2.12
2.19
1.82
1.11
1.01
1.44
1.12
0.46
0.92
0.86
1.27
1.59
0.98
1.24
0.79
0.68
0.66
0.85
0.48
0.80
2.85
0.30
0.96
1.26
0.92
0.67
1.45
1.45
Station
a
Depth (m)
T (°C)
Vent mouth.
1 m below vent mouth.
In-situ temperature measured with a thermocouple.
d
Data not available.
e
Absorption and fluorescence data for samples from 1 m below the yellow vent was invalid, since absorption coefficient increased much more rapidly with decreasing wavelength
when the wavelength was b 400 nm due to an unknown source of absorption.
f
SR was not calculated for some samples since some of the absorption values over the wavelength range of 350–400 nm were below detection limits.
g
C(1–3) were not determined for these three samples since they were identified as outliers during PARAFAC following the protocols of Stedmon and Bro (2008).
h
Fluorescence data for this sample from 3 m above the yellow vent might be biased by the low pH value and was excluded from the discussion.
b
c
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
unknown source of absorption, and they were excluded from the discussion (Fig. 2). The a300 values of those two samples (39.1 and
23.5 m − 1) were one order of magnitude higher than those in the
white vents and even higher than the maximum in Taiwan rivers
(19.5 m − 1 in the Yen-Shui River, Fig. 2).
2.5. Fluorescence EEM measurements, PARAFAC modeling, HIX and BIX
calculation
Previous studies reveal that, within the pH range of 5–9, fluorescence intensity of all fluorophores change by only 10% (Hudson
et al., 2007). All samples except the excluded two and another one
from 3 m above the yellow vent in the second cruise had a pH of
4.83–8.08 (Table 1) and hence were not adjusted for fluorescence
measurements. EEM spectra were measured using a Cary Eclipse fluorescence spectrophotometer with scanning emission spectra from
300–600 nm (every 2 nm) at an excitation of 240–450 nm (every
5 nm). Instrument performance was tested using the validation program of the instrument everyday before sample measurements,
which included the excitation and emission wavelength accuracy,
spectral bandwidth accuracy of excitation and emission slit, and
Raman water sensitivity. The scan rate was 1200 nm min − 1 under
ratio mode. The fluorescence spectra were corrected according to
the files provided by the manufacturer. Sample EEM spectra were
Raman calibrated and subtracted by Milli-Q water blank scanned on
the same day (Lawaetz and Stedmon, 2009). Samples were not corrected for inner filter effects because of the low CDOM absorption
for all the samples, except the excluded two from 1 m below the yellow vent mouth (A254 and A350 at 1 cm path length were within 0.18
and 0.01).
The EEMs were modeled using PARAFAC in MATLAB 7.5 and “the
N-way toolbox for MATLAB” (Stedmon and Bro, 2008). Fluorescence
at either excitation b250 nm or emission >500 nm were not used, in
order to avoid the interference of noise signals (Stedmon and Markager,
2005) and Rayleigh–Tyndall peaks, respectively. Split-half validation
was used to determine the number of components (Stedmon and Bro,
2008). Three samples were identified as outliers during PARAFAC
following the protocols of Stedmon and Bro (2008) and hence were
excluded from the modeling. PARAFAC decomposed the EEM spectra
into individual fluorescent components and the fluorescence intensity
of each component in each sample was represented by its maximum
fluorescence Fmax (RU, i.e. Raman units).
In addition, the maximum fluorescence intensities of traditionally
defined fluorescence peaks were calculated for comparison with the
PARAFAC results. The peak regions (excitation/emission) summarized
by Coble (2007) were used: UVC-excited humic-like fluorescence
peak A (260/400–460 nm); UVA-excited humic-like peak C (320–360/
420–460 nm); UVA-excited marine humic-like peak M (290–310/
Fig. 2. Comparison of absorption spectra for samples from the yellow vent mouth (Y),
1 m below the yellow vent mouth (Y-1, one example of the two excluded samples), the
white vent mouth (W), seawater (Y2, salinity 34.5), and river waters (R1 and R2, from
the Kao-Ping and Yen-Shui Rivers, Taiwan, unpublished results).
67
370–410 nm); tyrosine-like peak B (275/305 nm); and tryptophanlike peak T (275/340 nm).
HIX and BIX were calculated from the EEM spectral data for assessing the humification degree and source of DOM (Huguet et al., 2009;
Birdwell and Engel, 2010). HIX was the ratio of the area under the
emission spectra at 434–480 nm to that at 300–346 nm, at an excitation wavelength of 255 nm. BIX was the ratio of the fluorescence
intensity at emission 380 nm to that at emission 430 nm, at an
excitation wavelength of 310 nm.
3. Results
3.1. Temperature, salinity, pH and DOC
Temperature varied from 28 to 105 °C in this study (Table 1).
Much higher values were found for 1 m below the yellow vent
mouth (105 and 97 °C) and below the white vent mouth (41 and
58 °C) than those for other stations (28–33 °C, mean: 29 ± 1 °C) during both cruises. Temperatures for 1 m below the vent mouths in this
study fell in the ranges previously reported for yellow and white
vents in the study area (78–116 °C and 30–65 °C, Chen et al.,
2005a). In addition, temperatures generally decreased from 1 m
below the vent mouths to the sea surface.
Salinity varied within 32.8–34.1 (Table 1). The lowest values were
found for 1 m below the white vent during the two cruises (32.8 and
33.0) and might be explained by phase separation (Chen et al.,
2005a). Salinity for 1 m below the yellow vent mouth (33.1) was
also low in the second cruise. Other stations had a mean salinity
value of 33.8 ± 0.1.
The pH values ranged from 2.22 to 8.08 (Table 1). In the first
cruise, the lowest pH (2.82) was measured for 1 m below the yellow
vent mouth. In the second cruise, two low pH values (2.22 and 2.62)
were determined for 1 m below and 3 m above the yellow vent
mouth. With the exception of these three lowest values, other pH
values were in the range 4.83–8.08, which would have a limited
impact on the fluorescence results (Hudson et al., 2007).
In the first cruise, the DOC concentration was in the range
62–82 μM (Table 1). Notably, the DOC values at the two vent mouths
and 1 m below them (62–70 μM) generally fell within the range for
the other stations Y1, Y2, W1 and W2 (64–82 μM). This suggested
that the submarine hydrothermal vents off NE Taiwan were likely to
be neither a strong sink nor a strong source of DOC for the ambient
seawater. In the second cruise, the DOC concentration varied within
60–99 μM (mostly within 69–87 μM), with a mean value of 77 ±
7 μM. Similarly, Hung et al. (2003) report a DOC concentration of
75–85 μM for the Kuroshio water which dominates the seawater in
our study area.
3.2. Absorption coefficient and spectral slope ratio of CDOM
In this study, a300 was 0.08–3.58 m − 1 for all samples (Fig. 3).
However, except for the samples from the vents and their overlaying
water column, others had a300 b0.70 m − 1 with an average of 0.34 ±
0.12 m − 1. These are one order of magnitude lower than those for Taiwan Rivers (2.0–19.5 m − 1, unpublished data). Therefore, the CDOM
concentration was low in the seawater surrounding Kueishantao
islet, with the exception of samples affected by the hydrothermal
vents. This was likely due to the seawater in this area being dominated
by the Kuroshio current without local river discharge. Similarly, the
seawater around Orchid Island off SE Taiwan, which is also dominated
by the Kuroshio water, has a low CDOM absorption (a300: 0.38 ±
0.08 m− 1, n = 7, unpublished data).
The highest values of a300 were found at the white hydrothermal
vent mouth and 1 m below it during both cruises, ranging from
1.85–3.58 m − 1 (mean: 2.52 ± 0.88 m − 1, Fig. 3). They were lower
than those in some coastal seas with abundant terrestrial CDOM
68
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
although further direct examination of the DOM molecular weight
using other methods such as size exclusion chromatography is
needed.
3.3. PARAFAC components and their abundance
Fig. 3. Vertical distribution of the absorption coefficient (a300) (Y. 8/3 and Y. 8/24: yellow
vent and its overlying water column on August 3rd and 24th, with the vent mouth at a
depth of 8.5 m; W. 8/5 and W. 8/25: white vent and its overlying water column on August
5th and 25th, with the vent mouth at depths of 16 m on August 5th and 17 m on August
25th; others: all other stations).
inputs (e.g., 4.89 ± 0.19 m − 1 for the Baltic Sea, Stedmon et al., 2010).
However, they were ~6 times higher than those of the surrounding
seawater and comparable with those in CDOM-poor freshwater
such as the Kao-Ping River in Southwest Taiwan (Fig. 2). Therefore,
the white vents acted as a source of CDOM for the seawater.
In our first cruise, a300 was also higher at the yellow vent mouth
and 3 m above it (1.04 and 1.00 m − 1, Fig. 3). Therefore, the yellow
vent might also be a source of CDOM for seawater. However, a300 decreased to 0.43 m − 1 3 m above the vent in the second cruise. Possible
explanations are that the flow pathway of hydrothermal fluids could
be influenced by tides in the water column or that the plume occasionally spreads out before reaching the surface (Chen et al., 2005a).
Interference from other vents cannot be ruled out. As mentioned
above, two yellow vent samples were excluded.
SR is suggested to be negatively correlated with the molecular
weight of DOM (Helms et al., 2008; Ortega-Retuerta et al., 2010).
Our results for SR ranged from 1.56 to 2.14 for samples from the hydrothermal vent mouths and 1 m below them. These were within
the values for the continental shelf and slope waters adjacent to Delaware Bay (1.5–3.9) and higher than those for the estuarine waters in
Delaware Bay (0.9–1.3, Helms et al., 2008). In our first cruise, SR decreased from the vent mouth to the sea surface for both vents, a
trend which was also found for the white vent in the second cruise.
This suggested that CDOM in the vent fluids might have a lower molecular weight than in the overlying seawater (Helms et al., 2008),
Generally, there are two types of fluorophores in natural aquatic
environments: humic-like and protein-like. The former includes
UVC peak A (260/400–460 nm), UVA peak C (320–360/420–
460 nm) and UVA peak M (290–310/370–410 nm), while the latter
includes tyrosine-like peak B (275/305 nm) and tryptophan-like
peak T (275/340 nm) in the EEM fluorescence spectra (Coble, 2007).
In our study, three fluorescent components were identified using
PARAFAC (Fig. 4). C1 had two excitation maxima at ≤250 nm and
310 nm and one emission maximum at 454 nm. It resembled a combination of the traditionally defined peaks A and C (Coble, 2007)
and hence was a humic-like component, similar to C1 in Stedmon
and Markager (2005) and C1 in Kowalczuk et al. (2009). This was
also supported by the strong correlations between C1 and peaks A
and C (Table 2). C2, with excitation/emission maxima of 290/
358 nm, covered the EEM spectral region of peaks T and M (Coble,
2007), although the fluorescence intensity in the region of peak T
was dominant (Fig. 4). There were also strong correlations between
C2 and peaks T and M (Table 2). Both peaks T and M, and hence probably C2, could be generated by marine biological production
(Romera-Castillo et al., 2010; Omori et al., 2011; Guo et al., 2011).
C3 had excitation/emission maxima (275/≤300 nm) identical to
those of peak B (Coble, 2007) and the tyrosine-like component, e.g.,
C1 in Murphy et al. (2008); C4 in Yamashita et al. (2010); and C5 in
Jørgensen et al. (2011). C3 and peak B were also correlated (Table 2).
Overall, FDOM in the study area was characterized by abundant
protein-like components. Both the fluorescence intensities of C2
(0.031 ± 0.019 RU) and C3 (0.045 ± 0.016 RU) were comparable to
that of C1 (0.036 ± 0.010 RU) (Fig. 5). Furthermore, both C1 and C3
had the highest fluorescence intensity in the white hydrothermal
vent, suggesting the potential of the white vent as a source for both
humic-like and tyrosine-like FDOM (Fig. 5). In contrast, the fluorescence intensity of C2 in the hydrothermal vents was similar to the
others. This might be explained by the fact that both peaks M and T
could originate from autochthonous production in marine environments (Romera-Castillo et al., 2010; Jørgensen et al., 2011).
3.4. HIX and BIX
The HIX for the humic content of DOM has low values (b4) for
non-humified DOM of biological or aquatic bacterial origin, but high
values (>10) for DOM with a strong humic character or with an important terrigenous contribution (Huguet et al., 2009). The index
Fig. 4. EEM contours of the three fluorescent components identified using PARAFAC.
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
Table 2
Correlation coefficient (r) between the fluorescence intensities of PARAFAC components (C1, C2 and C3) and traditionally defined peaks (UVC humic-like peak A, UVA
humic-like peak C, UVA marine humic-like peak M, tryptophan-like peak T, and
tyrosine-like peak B) (Coble, 2007) (n = 55).
C1
C2
C3
A
C
M
T
B
0.85
0.60
0.71
0.80
0.50
0.63
0.39
0.94
0.31
0.53
0.88
0.58
0.62
0.07
0.77
ranged between 0.53 and 2.11 in our study, which indicated that the
DOM originated mainly from a biological or aquatic bacterial source
and had a low humic content (Table 1, Fig. 5). In particular, the four
samples from the white vent mouth and 1 m below it had lower
HIX values (0.93 ± 0.19) than the ambient seawater (1.51 ± 0.23 for
stations Y(1–2), W(1–2), A–E, G–J and 1–15) (F = 24.3 > F0.05 = 4.1,
using one-way analysis of variance). The other three low HIX values
b1.0 were found in the second cruise either from the water column
above the yellow vent or from the bottom water at station F, close
to the yellow vent. This suggested that the DOM in the hydrothermal
vents might be freshly-produced with a low humic content.
The BIX for the autochthonous contribution to DOM has low
values (b0.6–0.7) for DOM with low autochthonous components,
Fig. 5. Vertical distribution of the fluorescence intensity of C(1–3) and the humification
index (HIX) (Y. 8/3 and Y. 8/24: yellow vent and its overlying water column on August
3rd and 24th, with the vent mouth at a depth of 8.5 m; W. 8/5 and W. 8/25: white vent
and its overlying water column on August 5th and 25th, with the vent mouth at depths
of 16 m on August 5th and 17 m on August 25th; others: all other stations).
69
but high values (>0.8) for DOM with a strong autochthonous component of biological or aquatic bacterial origin (Huguet et al., 2009;
Birdwell and Engel, 2010). The index varied greatly from 0.3 to 4.0
in our study (Table 1), which might be partly due to the low fluorescence intensity at the excitation wavelength of 310 nm, which was
much longer than the 255 nm used for HIX. Similarly, BIX varied
within 0.5–3.0 for terrestrial cave and spring waters which have a
low HIX b5 (Birdwell and Engel, 2010). Overall, BIX had a mean
value of 1.27 ± 0.63 and only nine of the 60 values were b0.7. Therefore, the results of BIX in this study also suggested an overall strong
autochthonous contribution to DOM. Furthermore, the BIX values at
the vent mouths or 1 m below them were ≥0.93 in both cruises, indicating again that the DOM in the hydrothermal fluids originated
mainly from biological or aquatic bacterial activity (Huguet et al.,
2009; Birdwell and Engel, 2010).
4. Discussion
Generally, there are limited available DOC data for submarine hydrothermal vents worldwide, which has partly limited the assessment of carbon exchange between submarine hydrothermal vent
and the ocean. In our study, the DOC concentration at the two vent
mouths and 1 m below them (62–70 μM) fell within the range for
the ambient seawater, suggesting that the submarine hydrothermal
vents off NE Taiwan were likely to be neither a strong sink nor a
strong source of DOC for the ambient seawater. Similarly, the DOC
concentrations of two shallow submarine hydrothermal vents (56
and 72 μM) are comparable to that of the ambient seawater (69 μM)
in Vulcano, Italy (Skoog et al., 2007). For comparison, in the deepsea hydrothermal vents, DOC is much lower in high temperature
vents (8–24 μM) and off-axis hydrothermal fluids (7–27 μM), but
higher in low temperature diffuse fluids (34–71 μM) than the background seawater (36 μM) (Lang et al., 2006). The active removal
and addition processes for DOC in the deep-sea vents are affected
partly by microbial abundance (Lang et al., 2006).
In our study, the correlation between the a300 and DOC was weak
and scattered (Fig. 6), suggesting that CDOM might have a different
spatial distribution with DOC. Similarly, there is a lack of correlation
between CDOM absorption coefficient and DOC in the open ocean
(Nelson et al., 2010). The a300 values in our study (0.34 ± 0.12 m − 1)
were low for all samples except for those from the vents and their
overlaying water column. This low value for the background seawater
was comparable to those for rainwater (mean: 0.37 m − 1, Kieber et
al., 2006), but lower than those in the North Sea (0.60 ± 0.04 m − 1,
Stedmon et al., 2010). In contrast, a300 at the white hydrothermal
vent mouth and 1 m below it (2.52 ± 0.88 m − 1) was much higher
than in the surrounding seawater. The a300 value was also high at
the yellow vent mouth (1.04 m − 1) in our first cruise. These a300
values for the submarine hydrothermal samples were also comparable to those for the terrestrial springs and caves (1.6–12.0 m − 1,
mean: 3.0 ± 2.1 m − 1, calculated from a375 and the spectral slope in
Birdwell and Engel (2010)). Therefore, the submarine hydrothermal
vents off NE Taiwan acted as a source of CDOM for the ambient
seawater.
Three fluorescent components were identified in our study:
humic-like C1, tyrosine-like C3, and C2 as a combination of marine
humic-like and tryptophan-like components. Their EEM spectral
characteristics were comparable to previous studies as mentioned
above (e.g., Coble, 2007; Murphy et al., 2008; Yamashita et al.,
2010; Jørgensen et al., 2011), suggesting that some of the DOM components in the hydrothermal vents might be similar to those in other
aquatic environments. The fluorescence intensity of C(1–3) was not
correlated with DOC (Fig. 6). A multi-linear regression, taking DOC
as the dependent variable and all three fluorescent components as
independent variables, also yielded an insignificant correlation
(r = 0.17, p = 0.24). Both C1 and C3 had the highest fluorescence
70
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
Fig. 6. Correlation of the DOC concentration vs. the CDOM absorption coefficient (a300) and the fluorescence intensity of C(1–3).
intensity in the white hydrothermal vent, suggesting that the white
vent was likely to be a source of both humic-like and tyrosine-like
FDOM for the ambient seawater (Fig. 5). Similarly, Svensson et al.
(2004) demonstrate that most shallow hydrothermal sites in Vulcano
Island (Italy) have total dissolved amino acid concentrations 3–114
times higher than the local seawater. In contrast, the fluorescence intensities of C1 and C3 at the yellow vent mouth were similar to the
ambient seawater in our first cruise (Table 1, Fig. 5). In addition, C2
is a component that can be produced by marine phytoplankton
(Romera-Castillo et al., 2010) and its fluorescent intensity in
both the yellow and the white vents was also similar to that at
other stations. Therefore, the white vent was probably more important than the yellow vent as a source of FDOM for the ambient
seawater in the study area. However, only the fluorescent components of DOM could be determined by fluorescence spectroscopy
and the linkage of the fluorescence peaks to specific classes of
compounds is still putative (e.g., Yamashita and Tanoue, 2003;
Jørgensen et al., 2011). Further determination of specific classes
of organic compounds is required for better understanding the
role of hydrothermal vents as either a source or a sink for specific
DOM components.
The HIX and BIX are useful for assessing the humification degree
and the source of organic matter (Huguet et al., 2009; Birdwell and
Engel, 2010). The low HIX (1.42 ± 0.30) and the generally high BIX
(1.27 ± 0.63) for all samples in our study (including those from the
hydrothermal vents) suggested that the DOM was likely to have a
low humification degree and was mainly freshly-produced by biological or bacterial activity. Similarly, CDOM in terrestrial cave and spring
waters has a low HIX of b5 and a BIX of 0.5–3.0, suggesting a low humification degree and a microbial origin (Birdwell and Engel, 2010;
Fig. 7). Furthermore, both submarine hydrothermal vents (and the
ambient seawater) and terrestrial cave and spring waters were distinct from isolated humics and low-mid salinity estuarine waters,
sediment organic matter and porewater, all of which have a much
higher humification degree and an overall lower BIX (Fig. 7; Huguet
et al., 2009; Birdwell and Engel, 2010). This also demonstrated the
power of fluorescence spectroscopy for differentiating DOM of different humification degree and from different sources.
5. Conclusions
Studies were carried out on two shallow water submarine hydrothermal vents off the northeast coast of Taiwan. The white vent could
act as a source of CDOM for seawater. The humic-like C1 and the
tyrosine-like C3, but not C2, had their highest fluorescence intensity
in the white vent. The low HIX and the overall high BIX suggested
that DOM probably had a low humification degree and was likely to
be freshly-produced by biological or bacterial activity in the study
area. The bioavailability of DOM in the hydrothermal fluids should
be determined in future studies, and further studies on the interactions among DOM, trace metals, the microbial community and trace
gases would significantly improve our understanding of the biogeochemical processes taking place in the research frontier of submarine
hydrothermal systems.
Fig. 7. Comparison of the humification index (HIX) and the autochthonous index (BIX)
for different water types, including submarine hydrothermal vents and surrounding
seawater in our study, and literature data for estuarine waters (Huguet et al., 2009),
terrestrial cave and spring waters, mat extract, isolated humics, sediment organic matter
(OM), and tryptone (Birdwell and Engel, 2010).
L. Yang et al. / Marine Chemistry 128-129 (2012) 64–71
Acknowledgments
This work was financially supported by the National Natural Science
Foundation of China (No. 40810069004), Xiamen University Project
211-III (Overseas Visiting and Study Program for Graduate Students),
the National Science Council of Taiwan (NSC 98-2621-M-110-001-MY3)
and the “Aim for the Top” University Program of Taiwan. We thank
Bing-Jye Wang, Hsiang-Cheng Huang and His-Hsiang Lin for their help
in sampling, and Chun-Hung Yeh for his help in pH measurement.
Professor John Hodgkiss is thanked for his assistance with English.
Dr. W. Miller and two anonymous reviewers are thanked for their
comments that greatly improved the quality of the paper.
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